This invention relates to systems for maintaining the excitation of a field winding of alternating current generator (alternator), and more particularly to systems for maintaining said excitation during excessive output current conditions.
The voltage and current for the field winding of an alternator is often provided by a shunt-connected, automatic voltage regulator (AVR). Such a regulator receives its power from the alternator and in turn supplies power either directly to the alternator's field winding or to the field winding of an exciter. In the latter case the output of the exciter is in turn supplied to the alternator's field winding. The power supplied by the AVR is a function of the generator output voltage and, sometimes, the generator frequency. Under excessive output current conditions of the generator (such as a fault or overload on one or more generator output lines) the output voltage of the generator drops severely. Ideally, it is desirable for the generator to maintain a short circuit current of about three times rated full load current for at least a time interval sufficient for circuit breakers to clear faults on the lines. Although the regulator attempts to increase the excitation of the field winding under these conditions to provide the desired short circuit current, the low voltage on the generator output lines often results in insufficient power being supplied to the voltage regulator. Consequently the field excitation actually drops and the generator output collapses. A related problem with shunt-regulated alternators occurs during motor starting operations. During motor starting the output voltage of the alternator falls, which lenghtens the amount of time required to start the motor.
To prevent collapse of the generator output, regulating systems which supplement or boost the automatic voltage regulator under excessive output current conditions have been developed, including that disclosed in U.S. Pat. No. 3,316,479. The system of that patent uses a saturable transformer having a primary winding connected in series with an output line of the generator (see FIG. 1, transformer 16). The output of the secondary of said transformer is supplied to a voltage regulator (regulator R1), said output of the secondary being a continuous source of power for the regulator during excessive output current conditions (see col. 3, lines 9-43). The saturable transformer is designed to limit the effective supplemental voltage supplied to the regulator. However, expensive high voltage rectifiers are used with the system of that patent and, in general, with systems which use saturable transformers to limit the supplemental voltage.
Reference is also made to copending U.S. patent application Ser. No. 22,961, filed Mar. 23, 1979, now U.S. Pat. No. 4,264,856, which discloses a system for maintaining the excitation of an alternating current generator during excessive output current conditions. The system has booster stages each of which utilizes a current transformer to develop a voltage from the current in a respective generator output line. During excessive output current conditions, a sensing circuit causes the booster stages to impress their generated voltages upon the generator field winding. The power supplied by the booster stages is supplemental to that supplied by the regulator.
Prior art three-phase generator regulators which receive power from only one line-to-line phase of the generator, such as shown in the above-mentioned application Ser. No. 22,961, now U.S. Pat. No. 4,264,856, have an output voltage which is unpredictable under excessive output current conditions because its output voltage is determined by the current and voltage in only the particular line-to-line phase from which it receives power. For example, a short in the line-to-line phase from which the regulator receives its power will reduce the regulator output drastically while a similar short in a different line-to-line phase may result in little or no change in regulator output. Therefore, regulator output is an unknown, which fact makes it difficult for an excitation maintaining, or boost, system used in conjunction with such a regulator to supply the proper field excitation power. This problem is compounded by the fact that different fault conditions (line-to-line shorts, line-to-neutral shorts, three-phase shorts) require different field excitations. It is desirable that the boost system supplement the excitation the required amount to provide the desired short-circuit current regardless of the generator phase affected and regardless of the particular fault condition.
Many automatic voltage regulators have a frequency responsive transfer characteristic. If the prime mover slows and the generator frequency thereby decreases, the regulator as a protective measure decreases the field winding excitation to decrease the generator output voltage. Such a decrease in generator voltage may be treated as a fault or overload and boost the field, thereby defeating the protection afforded by the regulator.